This invention relates generally to a high-frequency heating device commonly called "microwave oven or electronic oven" in which high-frequency induction heating is used for uniformly heating an object to be heated, such as food, and more particularly to such a heating device of the type in which the degree of uniform heating, for example, of a piece of frozen sea food or frozen meat (which most particularly need to be uniformly heated) by high-frequency electromagnetic waves within a heating chamber can be increased.
High-frequency heating used for thawing frozen food is characterized in that the time required for thawing is short; however, in the high-frequency heating, the degree of heating is about 1,000 times higher in a thawed condition than in a frozen condition, and therefore if a certain portion of the frozen food is thawed, this portion is excessively heated to be boiled whereas the other portions still remain frozen. This is a disadvantage of the high-frequency heating. Therefore, unless the degree of the high-frequency heating is kept uniform, a satisfactory thawing performance can not be obtained.
Many methods of uniformly heating an object to be heated within a heating chamber have heretofore been proposed. These includes a stirrer fan-type in which high-frequency electromagnetic waves are stirred by a stirrer fan, a rotary table-type in which the object to be heated is rotated by a rotary table, and a type in which an antenna for radiating electromagnetic waves is rotated. There is also a type having the features of the stirrer fan-type and the rotary table-type.
A typical example of such arrangement is disclosed in Japanese Patent Examined Publication No. 60-25875, in which a rotary table is mounted on a bottom of a heating chamber or oven compartment, and a stirrer fan is provided at the rear of the heating chamber so as to uniformly heat an object to be heated.
More specifically, as shown in FIG. 9, the rotary table 34 is mounted on the bottom of the heating chamber 33. A wave stirrer fan 37 of metal is mounted on a convexly-drawn portion 36 of a rear wall 35 of the heating chamber 33, and electric waves, radiated or applied into the heating chamber 33 through a wave guide 39 mounted on a top wall 38 of the heating chamber 33, are stirred by the wave stirrer fan 37, and in this condition the rotary table 34 is rotated to rotate the object 40 to be heated so as to uniformly heat the same.
In this arrangement, however, all of the high-frequency electromagnetic waves radiated into the heating chamber 33 are not stirred by the wave stirrer fan 37, and therefore the stirring effect is not adequate. And besides, since the wave guide 39 is provided on the top wall 38 of the heating chamber 33, an upper portion of the load tends to be heated more strongly than the other portions. Moreover, in the type of heating cooking device which has electric heaters provided respectively at the upper and lower sides of the heating chamber, it is difficult to provide the upper heater of a satisfactory construction because of the provision of the wave guide 39, which results in a problem that a sufficient distribution of heat by the heaters could hardly be obtained.
In another conventional heating device as disclosed in U.S. Pat. No. 2,748,239, an opening for admitting high-frequency electromagnetic waves is provided in a rear wall of an oven compartment, and wave stirrer blades are mounted within a wave guide connected to the opening, and the wave stirrer blades are rotated so as to make uniform the electric field of the high-frequency electromagnetic waves within the oven compartment; however, since the opening is provided at the rear side of the oven compartment, a rear portion of an object to be heated, such as food, tends to be inevitably heated strongly. This tendency is not eliminated even if the high-frequency electromagnetic waves are stirred by the wave stirrer blades. Therefore, in the thawing which requires a high degree of uniformity of the high-frequency electromagnetic waves, part of the food is boiled whereas the other parts remain frozen.
U.S. Pat. No. 4,136,271 discloses a method in which an antenna for radiating high-frequency electromagnetic waves, as well as a wave stirrer fan for stirring the high-frequency electromagnetic waves, is provided at an upper side, and a rotary table is provided at a bottom side. In this method, since the antenna and the wave stirrer fan are disposed close to each other, a sufficient stirring effect is achieved by the wave stirrer fan; however, because the electromagnetic wave supply portion is disposed at the upper side, it is difficult for the wave energy to reach the central portion of the rotary table, and it is difficult to obtain a balance of heating between the central portion and outer peripheral portion of the rotary table, and the outer peripheral portion tends to be heated more strongly than the central portion. As a result, a uniform heating has not been achieved. Where the electromagnetic wave supply portion is provided at the upper side, the electromagnetic waves supplied from the upper side are once reflected by the side or peripheral wall of the oven compartment, and then reach the surface of the rotary table. Therefore, when the size of the oven compartment or the position of the wave supply portion is changed slightly, the degree of heating in the vicinity of the surface of the rotary table is greatly changed. Furthermore, there has been encountered another disadvantage that when milk or the like held in a cup is warmed, an upper portion thereof is inevitably heated strongly.
In any of the above-mentioned methods, the distribution of the electromagnetic waves has not been uniform enough to enable a uniform thawing of the frozen food. In the general heating other than the thawing, unevenness in heating has also been encountered.